1. Field of the Invention
The present invention relates to a method of fabricating single-crystal substrates of silicon carbide (SiC).
2. Description of the Prior Art
SiC has many crystal structures (called "polytype") and therefore has forbidden band gaps of 2.2 to 3.3 electronvolts (eV) depending on the crystal structure. SiC is very stable thermally, chemically and mechanically and has resistance to damage due to radiation. The material can be of the p-type and n-type both with good stability, which is a rare case with wide-gap semiconductors. Accordingly SiC appears useful as a semiconductor material for electronic devices operable at high temperatures or with great electric power, for highly reliable semiconducor devices, radiation-resistant devices, etc. Further SiC will provide electronic devices which are usable in an environment where difficulties are encountered with devices made of conventional semiconductor materials, thus greatly enlarging the range of applications for semiconductor devices. Making use of its wide energy gap, SiC is also usable as a semiconductor material for optoelectronic devices for the visible light of short wavelengths and near-ultraviolet light. Whereas other wide-gap semiconductors usually have a heavy metal contained as the main component and therefore entail the problems of pollution and resources, SiC is free of these problems and therefore appears to be a promising electronic material.
Despite these many advantages and capabilities, SiC has not been placed into actual use because the technique still remains to be established for growing SiC crystals with good reproducibility which is required for commercially fabricating SiC substrates of large area with high quality and high productivity.
Conventional processes for preparing SiC single-crystal substrates on a laboratory scale include the so-called sublimation method [also termed the "Lely method"; "Growth Phenomena in Silicon Carbide" W. F. Knippenberg: Philips Research Reports, Vol. 18, No. 3, pp. 161-274 (1963). (Chapter 8 "The Growth of SiC by Recrystallization and Sublimation", pp. 244-266)] wherein SiC powder is sublimed in a graphite crucible at 2,200.degree. C. to 2,600.degree. C. and recrystallized to obtain an SiC substrate, the so-called solution method ["Growth of Silicon Carbide from Solution" R. C. Marshall: Material Research Bulletin, Vol. 4, pp. S73-S84 (1969)] wherein silicon or a mixture of silicon with iron, cobalt, platinum or like impurities is melted in a graphite crucible to obtain an SiC substrate, and the Acheson method ["Growth Phenomena in Silicon Carbide" W. F. Kinippenberg: Philips Research Reports, Vol. 18, No. 3, pp. 161-274 (1963). (Chapter 2 "Preparative Procedures", pp. 171-179)] which is generally used for commercially producing abrasives and by which SiC substrates are obtained incidentally.
With the sublimation method and the solution method, a large number of crystals can be obtained, but it is difficult to prepare large SiC single-crystal substrates since many crystal nuclei occur in the initial stage of crystal growth, while the SiC product has several kinds of crystal structures (polytype). Thus, these processes have yet to be improved for preparing large SiC single crystals of single polytype with good reproducibility. Thus SiC substrate incidentally obtained by the Acheson method must be improved in purity and crystallinity for use as a semiconductor material, so that the process is not suited to commercial production of SiC substrates.
With improvements in semiconductor techniques in recent years, it has become possible to form a thin single-crystal film of 3C-type SiC (having a crystal structure of cubic system and an energy gap of 2.2 eV) on a foreign substrate of silicon (Si) which is available as a large-sized single-crystal substrate of good quality, by the heteroepitaxial technique with chemical vapor deposition (CVD) ["Some properties of Vapor Deposited SiC" K. E. Bean and P. S. Gleim: Journal of the Electrochemical Society, Vol. 114, No. 11, pp. 1158-1161 (1967). ".beta.-Silicon Carbide Films P. Rai-Choudhury and N. P. Formigoni: Journal of the Electrochemical Society, Vol. 116, No. 10, pp. 1440-1443 (1969). "Growth, Texture, and Surface Morphology of SiC Layers" K. A. Jacobson: Journal of the Electrochemical Society, Vol. 118, No. 6, pp. 1001-1006 (1971). "Vapor-Phase Deposition of Beta-Silicon Carbide on Silicon Substrates" K. Kuroiwa and T. Sugano: Journal of the Electrochemical Society, Vol. 120, No. 1, pp. 138-140 (1973)]. The CVD method is a fabrication technique having high productivity on a commercial scale and is a promising technicque for growing an SiC single-crystal film of large area and high quality over Si substrates with good reproducibility. Usually used in this method are SiH.sub.4, SiCl.sub.4, SiH.sub.2 Cl.sub.2, (CH.sub.3).sub.3 SiCl or (CH.sub.3).sub.2 SiCl.sub.2 as the silicon source, CCl.sub.4, CH.sub.4, C.sub.3 H.sub.8 or C.sub.2 H.sub.6 as the carbon source, and hydrogen, argon or the like as the carrier gas for epitaxially forming a thin single-crystal film of 3C-type SiC on an Si substrate which is heated to a temperature of 1,200.degree. C. to 1,400.degree. C.
However, SiC is not satisfactorily compatible (wettable) with the Si foreign substrate, while SiC differs from Si by as much as 20% in lattice constant, so that even if it is attempted to grow a single crystal of SiC directly on the Si substrate, SiC fails to grow into a layer of a single-crystal film but grows into a polycrystalline form of dendritic structure, or a very thin single-crystal film, if obtained, tends to become deteriorated and polycrystalline with an increase in its thickness.